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Old 2nd April 2012, 02:14 PM   #41
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Hi Matt,
Please forgive my ignorance, can you please show me on how you install a multiturn potentiometer in place of R8? As far as I know, they have 3 legs on it, do you use only 2 legs?
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Old 2nd April 2012, 02:32 PM   #42
6L6 is offline 6L6  United States
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Solder the wiper to one of the legs. That changes the pot into a variable resistor.
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Old 2nd April 2012, 09:17 PM   #43
slomatt is offline slomatt  United States
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Quote:
Originally Posted by TjongKristian View Post
Hi Matt,
Please forgive my ignorance, can you please show me on how you install a multiturn potentiometer in place of R8? As far as I know, they have 3 legs on it, do you use only 2 legs?
As 6L6 posted you just need to solder the wiper to either of the legs.

The way a potentiometer works is that there is a fixed resistance between the two outer pins, and the middle pin is connected to the wiper which moves between the outer legs. For example, if you have a 100 ohm linear pot then you'll measure 100 ohms between the outer legs, and the resistance between the wiper and either leg will vary from 0-100 ohms as you adjust it. This allows you to use the pot as a voltage divider with two resistances that are inversely related. If you short the wiper to one of the legs you'll have a variable resistance between the wiper and the other leg.

In this particular case you just want to find a potentiometer where the outer two legs are spaced properly to fit on the board, and then you bend the wiper (center pin) and solder it to one of the legs.

I used the following parts and they fit just fine.

Bias adjustment (replaces R13)
http://search.digikey.com/scripts/Dk...00K-ND&x=0&y=0

DC offset adjustment (replaces R8)
http://search.digikey.com/scripts/Dk...=SP064W-500-ND

- Matt
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Old 2nd April 2012, 10:43 PM   #44
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I just bent one of the outer pins out of the way and soldered the other outer pin and wiper into the PCB.......
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Old 2nd April 2012, 10:48 PM   #45
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Thank you for the answers and advices.
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Old 3rd April 2012, 10:32 AM   #46
Atilla is offline Atilla  Norway
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By the way - for those blue pots, I strongly advice you use some solid glue, like silicon when you install it on the board. With some models the pin connections aren't strong enough and after adjusting them a couple of times they might bend and snap out of the blue casing. A dot of hot glue fixes that.
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Old 4th April 2012, 07:05 AM   #47
slomatt is offline slomatt  United States
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This evening I temporarily put an additional 1.5k ohm resistor in parallel with each existing 1.5k ohm R14 and R16 resistors so I could test how the circuit would behave if those were 750 ohm instead. With this setup I was able to increase the bias to 1.25A before it maxed out. Based on this I am going to leave the stock R14/16 in place and run ~1.2A bias for now. If I need to go higher I will replace the 0.47 ohm source/sense resistors with 0.33 ohm resistors. Assuming the current source can maintain the same voltage range across R28 this should make a 1.5A bias easily attainable.

While I had the boards out of the amp I also bypassed C1/2/3 with 0.01uF 63v Vishay KP18303 capacitors. I haven't had a chance to listen these yet, but hope they make a noticeable improvement.

- Matt
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Old 10th April 2012, 02:52 AM   #48
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Matt, have you checked your amp with a 10Khz square wave? Mine looked quite sub-optimal, and with the assistance of Zen Mod I dialed in the value of C4 (20 pf) and improved the measured performance appreciably.

The gory details start at post 79 in this thread:

Aleph 30 the best choice of C1 C2 C3 on brian gt PCB
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Old 10th April 2012, 04:56 PM   #49
slomatt is offline slomatt  United States
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Quote:
Originally Posted by boywonder View Post
Matt, have you checked your amp with a 10Khz square wave? Mine looked quite sub-optimal, and with the assistance of Zen Mod I dialed in the value of C4 (20 pf) and improved the measured performance appreciably.

The gory details start at post 79 in this thread:

Aleph 30 the best choice of C1 C2 C3 on brian gt PCB
Not yet, but I did see your discussion with Zen Mod a few days ago and it is on my to do list. I have some large 8 ohm resistors on order to use as a dummy load. Thanks for pointing this out!

- Matt
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Old 12th April 2012, 07:02 AM   #50
slomatt is offline slomatt  United States
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I dug up some 10w 3.3ohm resistors and made a 6.6ohm dummy load to test the amp but quickly ran into a problem. I used a signal generator on my laptop and was unable to get a clean 10kHz square wave using either the built in DAC or an external one, as the frequency increased the waveform on my scope looked more and more like a sine wave instead of a square wave. I checked on both my DSO Nano V2 (200KhZ) and a not-quite-properly-working 35MHz scope and saw the same results on both. So, this makes high frequency testing very difficult.

I was able to test using a 1kHz square wave and had the following results. Due to limitations of the signal generator I was testing at a low voltage. Interestingly the signal directly out of the laptop showed the same noise you see on the amp output shown on the Nano.

First off here is the amp output of a 1khz square wave displayed on the DSO Nano. As you can see there is a lot of noise.
Click the image to open in full size.

My full-size scope is having issues at the moment, but I was able to get a semi-clear read. This seems to show ringing on the 1kHz signal.
Click the image to open in full size.

So, based on the last image it does look like more tweaking is in order. Before that I'm going to try to get a better test signal so I can get cleaner measurements.

- Matt
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